Separate Selection and Ctrl Z
I know this is an old post but I came up against this very same problem myself today while using 2.82 and was able to find the a way to replicate the result Chris Plush had, so it's likely this post will come up in a google search in the future :).
Select the vertices hit Shift + D to duplicate them and right-click to keep them in place.
Press P to separate.
When you Tab out of edit mode you will have the smaller mesh for the front side panel and the side guide mesh will have remained intact.